This invention relates generally to atomic or molecular frequency standards and more particularly to improvements in an atomic hydrogen maser.
Atomic or molecular devices, for example the hydrogen maser, the cesium beam standard, the rubidium cell resonance standard, and the rubidium maser have very precise and stable frequencies of operation which are accurately related to quantum transitions in atoms or molecules. The stability and precision of these devices depends upon the interaction of the atom or molecule with oscillatory electromagnetic fields which are produced within a microwave cavity resonator wherein the atom or molecule is present while undergoing those quantum transitions. The oscillatory electromagnetic field in the microwave cavity has essentially the same frequency and wavelength as the atomic or molecular transition radiation, and the physical size of said microwave cavity in each type of device (in the present state of the art) is directly related to the wavelength of the radiation. In an apparatus such as the atomic hydrogen maser, wherein the hydrogen transition wavelength is nominally 8.3 inches (21.1 Cm), the resultant cavity is typically cylindrical having a 10 inch (25.4 Cm) diameter and being 20 inches (50.8 Cm) in length. The cavity is invariably made of massive, stable material, such as fused silica, copper, or aluminum, and is enclosed within a large high vacuum envelope, additionally surrounded by large, multi-layer, magnetic shields and thermostat structures. The net result is that all past state-of-the-art hydrogen masers are very large, typically between 4 and 6 feet high and 2 feet deep by 2 feet wide; the entire devices have been very heavy, weighing typically between 400 and 800 pounds.
The atomic hydrogen maser is well known to provide one of the best, most stable and accurate, frequency standards presently known; however, the large size, weight, and consequent cost have been universally recognized as the predominant disadvantage in its use.
Ramsey (U.S. Pat. No. 3,255,423) disclosed an atomic hydrogen maser, and the illustration thereof indicates the elongated nature of state-of-the-art devices. A. O. McCoubrey, in his article "A Survey of Atomic Frequency Standards," Proceedings of the IEEE, February 1966, pages 124, 133, referred to size, weight and cost as the principal limitations to usefulness of the hydrogen maser. Peters (U.S. Pat. No. 3,924,200) invented a storage bulb which allowed accurate measurement of "wall shift" but did not disclose any matters anticipating the present invention.